In a large punch bowl, combine rum and Ajax syrup and stir.
Add ice to individual glasses and fill glasses ¾ with punch.
Top glass off with chilled sparkling wine and stir quickly.
Garnish with watermelon spear and mint sprig.
To make Ajax Syrup (makes 2 liters):
Make 1 liter of simple syrup by adding 4 ¼ cups of white sugar to 4 ¼ cups warm water.
Stir until dissolved and set aside to cool.
Combine 14 cups pureed watermelon with 7 cups pureed cucumber.
Save the watermelon rind for garnishes.
Combine melon/cucumber puree with simple syrup